A well known liquid-liquid separation technique involves distillation whereby liquids of different boiling points can be separated For liquids having close boiling points, separation efficiency is enhanced by using distillation columns. Distillation is unsuitable for many types of liquids including temperature-sensitive liquids (i.e., bio-fluids, certain liquid food products), highly flammable liquids, such as the low molecular weight ethers, liquids which have toxic vapours, or liquid-liquid mixtures where separation must be done quickly.
Membrane separation is used where separation of liquid mixtures by distillation is undesirable. Membrane separation is more commonly used to remove liquids from a liquid/dissolved solid mixture. The methodology behind membrane separation can be very complex and can include osmotic pressure, relative vapour pressures, diffusion co-efficients, molecular size, molecular shape and molecular charge.
A known type of liquid-liquid separation using membranes involves a combined permeation/evaporation method. A liquid-liquid mixture is passed into a first chamber which is separated from a second chamber by a membrane. A pressure differential is applied across the membrane and by correct choice of the membrane, a particular component in the liquid-liquid mixture will permeate across the membrane selectively in preference to other components. This type of membrane separation is described in European patent specification 034G739 and Japanese patent specification 1-159007. While this technique allows separation of temperature-sensitive components, a disadvantage with the technique is that it is relatively slow, making it less suitable for separation of mixtures which are unstable over time, or which must be treated quickly. For instance, slow membrane separation techniques are not suitable for purification of bio-fluids which must be removed from a patient's body, purified and re-introduced into the patient's body, as quickly as possible. The slow membrane separations are also unsuitable in many pharmaceutical, medical and beverage applications where it is desirable to achieve separation as quickly as possible.
Tests have shown that throughput across a membrane can be increased by heating the membrane. However, heating can cause deterioration of temperature sensitive liquids as mentioned above.